A GaN-HEMT (high electron mobility transistor) is promising as a high power switching device from high breakdown electric field intensity and high mobility of GaN. Here, a thin insulating layer is provided directly underneath the gate to drive the GaN-HEMT by a voltage of the order of several volts generated by an IC (integrated circuit). If a high voltage is applied between the source and the drain, the thin insulating layer is easily broken. In other words, the withstand voltage of the GaN-HEMT itself is not high.
To cope therewith, a semiconductor device having a field plate (FP) on the GaN-HEMT has been proposed (hereafter referred to as GaN-FP-HEMT). According to the GaN-FP-HEMT, the withstand voltage of the GaN-HEMT relative to the source-drain voltage is increased to several hundred volts. (For example, refer to Wataru Saito, “Field-Plate Structure Dependence of Current Collapse Phenomena in Hight-Voltage GaN-HEMTs”, IEEE Electron device, Vol.31, July, 2010, No.7, pp.559-661, July 2010.)